Comparison of various noise mitigation technique used

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
__________________________________________________________________________________________
Volume: 02 Issue: 09 | Sep-2013, Available @ http://www.ijret.org 279
COMPARISON OF VARIOUS NOISE MITIGATION TECHNIQUE USED
WITH CLIPPING FOR REDUCTION OF PAPR IN OFDM
Hemant Choubey1
, Aparna Singh2
, Amit Shukla3
1, 3
Student, 2
Asst Professor, Electronics and Communication, UIT, M.P, India
hemantchoubey271986@gmail.com, aparna.kushwah@gmail.com, amit.rgi@gmail.com
Abstract
A simple technique used to reduce the PAPR of OFDM signals is to clip the signal to a maximum allowed value, at the cost of BER
degradation and out-of-band radiation. Clipping does not add extra information to the signal and high peaks occur with low
probability so the signal is seldom distorted. Out-of-band radiation can be reduced by filtering at the transmitter, the filter used in this
project consists on a FFT-IFFT pair which is easier to implement than traditional FIR filters and allows the implementation of the
clip & filter set several times in order to reduce the peak re growth that filtering introduces. The BER degradation can be mitigated by
reconstructing the signal at the receiver. We analyzed the performance of the decision-aided reconstruction (DAR) and improved DAR
(IDAR) techniques that iteratively try to guess the original symbols and proposed an improvement for one of those techniques.
Index Terms: Complementary cumulative distribution function (CCDF),high power amplifier (HPA), Orthogonal
Frequency Division Multiplexing (OFDM), Peak-to-Average Power Ratio (PAPR), inter-symbol interference (ISI)
-----------------------------------------------------------------------***-----------------------------------------------------------------------
1. INTRODUCTION
Introducing a filter after the clipping operation helps
mitigating the out-of-band radiation introduced by clipping.
Filtering, however, distorts also the signal and causes peak re
growth [6]. A common filter used for this purpose is the FIR
filter. In this application the filter used is the FFT-IFFT filter
proposed by Armstrong in [1]. This filter achieves better
performance than the FIR filter, it introduces less noise into
the signal, causes less peak regrowth and needs less
computation; since it operates symbol by symbol it causes no
inter-symbol interference. Clipping too large peaks is a simple
solution to the PAPR problem. Clipping belongs to the group
of techniques that reduce large peaks by nonlinearly distorting
the signal [8]. It does not add extra information to the signal
and too large peaks occur with low probability so the signal is
seldom distorted. The maximum peak power allowed is
determined by the system specifications, usually by the linear
region of the power amplifier. A maximum peak amplitude A
is chosen so that the OFDM signal does not exceed the limits
of this region, symbols that exceed this maximum amplitude,
will be clipped. The clipping function is performed in digital
time domain, before the D/A conversion as shown in Figure 1
and the process is described by the following expression
, 0 (1)
Where xck is the clipped signal, xk is the transmitted signal, A
is the clipping amplitude and f (xk) is the phase of the
transmitted signal xk. The graphical expression of this function
is shown in Figure 2. The clipping ratio (CR) is defined as
C.R = (2)
Fig1.Clipping in the transmitter
Fig2.Clipping Function

__________________________________________________________________________________________
Clipping is a non-linear process so it introduces in-band
distortion (Figure 3), also called clipping noise, and out-of-
band radiation and inter-carrier interference (Figure 4), which
degrade the system performance and the spectral efficiency.
The clipping noise is related to the difference between the
original signal xk and the clipped signal . The signal sent to
the receiver is the clipped signal, which is different from the
signal that we actually wanted to send. This difference is
measured by the signal to clipping noise ratio (SCNR)
SCNR = (3)
Fig3.SCNR measured for different number of subcarrier
Fig4.Out of band radiation
2.NOISEMITIGATION
After clipping the signal, the out-of-band radiation caused by
clipping falls in the zeros and then the signal is passed to the
FFT filter. The FFT function transforms the clipped signal
to frequency domain yielding . The information components
of are passed unchanged to the IFFT block and the out-of-
band radiation that fell in the zeros is set back to zero. The
IFFT block of the filter transforms the signal to time domain
and the obtained signal is passed to the D/A converter.
After filtering, the signal suffers peak regrowth so in order to
minimize this effect the clip & filter process can be repeated
several times.
3. NOISE MITIGATION METHODS
3.1 Decision-Aided Reconstruction (DAR) Method
The goal of the clipping operation is to reduce the PAPR of
the original signal xk; it introduces, however, some distortion
that makes the recovering of xk in the receiver more difficult.
The DAR method was proposed by Kim and Stuber in [5] in
order to mitigate the effects of the distortion introduced by
clipping [5].It is a nonlinear iterative reconstruction technique
implemented in the receiver. To implement this technique, the
receiver needs to know the clipping ratio A used at the
transmitter. The signal sent by the transmitter in frequency
domain can be expressed as
Yn = anXn+Cn (4)
Where αn is the clipping distortion, which changes randomly
from block to block and Cn are the noise introduced by
lipping. The received signal is therefore
Zn = hnYn+Wn = hnanXn+Qn (5)
Where hn is the complex channel gain, that can be accurately
estimated, and Qn is the sum of the additive white Gaussian
noise (AWGN) and the clipping noise Cn. In all the
simulations performed in this project ideal channel estimation
is supposed . Figure 5 shows the block diagram of the DAR
method, where rk is the received signal Zn in time domain.
Fig5. DAR algorithm block diagram

__________________________________________________________________________________________
If the CR is too low (this depends on the modulation), the
performance of DAR is degraded because false detection
increases.
3.2 Improved Decision-Aided Reconstruction (IDAR)
Method
The DAR method was proposed to mitigate the clipping noise
but it does not deal with the intermodulation noise . The inter-
modulation products are reduced by setting the IBO of the
power amplifier at the linear region; however, in order to
maximize the power efficiency of the amplifier, the IBO is set
nearly at the saturation region degrading this way the BER due
to the high non-linearity. The improved DAR method
mitigates both the clipping noise and the inter-modulation
noise. The IDAR method [2], like the DAR one, is an iterative
method applied at the receiver and was proposed by
Boonsrimuang et al. in [2], Figure 6 shows the block diagram.
It repeats at the receiver the clipping and the amplifying
operations, so the receiver needs to know the clipping
amplitude A and the amplifier characteristics. In Figure 6 can
be seen that the method starts by estimating the original signal
like in the DAR method
Fig6. IDAR algorithm block diagram
By using this method the intermodulation noise is mitigated,
relatively even when the nonlinear amplifier is operated at the
saturation region, while with DAR the BER is severely
affected by this noise. Although the method shows a good
performance for different required signal to noise ratios, for
CR lower than 3 dB around 0 dB.
3.3 IDAR Including the FFT-IFFT Filter (IDARF)
The IDAR method shows a very good performance by
mitigating the clipping and the intermodulation noises.
However, in order to reduce the out-of-band radiation it is
advisable to use a filter in the transmitter; in this case the filter
used is the FFT-IFFT filter. So, if the clipping operation is
followed by the filter at the transmitter, this filter should also
be included in the IDAR method at the receiver, Figure7
shows a block diagram of the new method. In order to include
the FFT filter in the IDARF method, the same number of zeros
as in the transmitter are inserted into the estimated signal
before converting it to time domain. Clip &filter is applied to
the time domain signal which is then amplified like in the
IDAR method. The zeros are removed from the signal after the
amplifier; the signal is transformed to frequency domain, the
zeros are removed and the signal is transformed back to time
domain in order to calculate the error signal. The error signal
is the difference between the estimated signal (before
adding the zeros) and the signal that suffered the clip&filter
and amplifying process, , after removing the zeros. The red
boxes in Figure7 show the operations that have to be added to
the IDAR algorithm in order to include the FFT filter in the
method.
Fig7. IDARF algorithm block diagram
4. SIMULATION RESULTS
After theoretically studying noise mitigation techniques, these
techniques have been tested both in a flat fading channel and
in a selective fading channel. The simulations have been made
for the 16-QAM and the 64-QAM modulations and all them
use 256 subcarriers, when analyzing the system for different
CR, the SNR used varies depending on the modulation, this
information is available in Table 3. The frequency selective
channel parameters are shown in Table 1 and the number of
iterations used for each method is shown in Table 2.
Table 1: Channel parameters
tap delay tap coeff
h0 0.0 0.8405
h1 0.3 0.4726
h2 1.0 0.2658

__________________________________________________________________________________________
Table 2: number of iterations
tap delay
clip&filter 1
DAR 2
IDAR 2
IDARF 2
Table 3: system parameters
number of subcarriers 256
CR 4dB
SNR 16QAM 15dB
SNR 64QAM 20dB
4.1 Flat Fading Channel
This section shows the performance of the BER of OFDM
signals modulated with 16 and 64QAM and in a flat fading
channel when varying the SNR and the CR .Figures 8, 9 and
10 show the BER performance of a 16QAM modulation when
varying the SNR from 0 to 18 dB.The simulations in Figure 8
show the performance of the DAR and IDAR methods when
no filter is applied at the transmitter. The results show that the
BER performance for DAR and for IDAR is very similar,
being IDAR slightly better. When the FFT filter is used at the
transmitter, IDARF shows clearly better performance than
DAR and IDAR. Figure 10 shows that IDARF is also better
than IDAR without filter at the transmitter, keeping the BER
very close to that of the no clipping case.
Fig 8: DAR and IDAR methods with no filter in the
transmitter, 16QAM modulation in flat fading channel, CR=4
dB, N=256 subcarriers
Fig 9: IDARF, DAR and IDAR methods with filter in the
transmitter, 16QAMmodulation in flat fading channel, CR=4
Fig 10: DAR, IDAR and IDARF methods with and without
filter in the transmitter,16QAM modulation in flat fading
channel, CR=4 dB, N=256 subcarriers
Figures 11, 12 and 13 show the same simulations as the
previous figures but this time holding the SNR at 15 dB and
varying the CR from 2 to 14 dBs. For CR > 4 dB, the
performance of DAR and IDAR in Figure 11, when no filter is
applied, is approximately the same, for CR< 4 dB performs
better than DAR. When the filter is applied, Figure 12, and for
CR < 8 dB approximately, IDARF provides a significant
better performance than DAR or IDAR and DAR provides no
improvement compared to just clip&filter, also the difference
between IDARF and DAR or IDAR without filter in the
transmitter is very small, see Figure 13. As the CR decreases,
the performance of all methods degrades, being DAR without
filtering at the transmitter the one that faster degrades. From a

__________________________________________________________________________________________
CR of 8 dB the performance of all methods becomes flat and
similar for all them.
Fig11: DAR and IDAR methods with no filter in the
transmitter, 16QAM modulation in flat fading channel,
SNR=15 dB, N=256 subcarriers
SNR=15 dB, N=256 subcarriers
Fig13: DAR, IDAR and IDARF methods with and without
filter in the transmitter, 16QAM modulation in flat fading
channel, SNR=15 dB, N=256 subcarriers
Figures 14, 15 and 16 show the BER performance of a
64QAM modulation when varying the SNR from 0 to 30 dB
and for a clipping ratio of 4 dB. In Figure 4.7, when no filter is
applied at the transmitter, the IDAR method performs clearly
better than DAR for SNR> 12 dB approximately, for lower
SNR their performance is nearly the same. For an SNR < 20
dB they behave like when no clipping is applied and after that
they behave like when just clipping is applied. When the filter
is used in the transmitter, Figure 4.8, the performance of
IDARF for the whole SNR range is very close to that when no
clipping is applied, on the other hand, there is nearly no
difference between using IDAR and using just clip&filter.
When comparing all methods in Figure 4.9, IDARF performs
clearly better than DAR and IDAR without filter in the
transmitter.
transmitter, 64QAM modulation in flat fading channel, CR=4
transmitter, 64QAMmodulationin flat fading channel, CR=4

__________________________________________________________________________________________
filter in the transmitter,64 QAM modulation in flat fading
channel, CR=4 dB, N=256 subcarriers
Figures 17, 18 and 19 show how those methods behave when
holding the SNR at 20 dB and varying the CR from 2 to 14
dB. Without filter in the transmitter, Figure 17, IDAR
performs slightly better than DAR for CR < 5 dB, after that
the performance is approximately the same. With filter in the
transmitter, 18, IDARF performs much better than the other
methods for CR < 9 dB and using DAR yields the same results
as just clip&filter. Regarding the whole set of methods, Figure
19, IDARF performs better than IDAR and DAR for CR < 6
dB.
SNR=20dB,N=256 subcarriers
transmitter, 64QAM modulation in flat fading channel
filter in the transmitter, 64QAM modulation in flat fading
channel, SNR=20 dB, N=256 subcarriers
CONCLUSIONS
When using the 16QAM modulation, DAR and IDAR have
similar performance when no filter is applied at the
transmitter, this performance is also very similar to that of
IDARF when the FFT filter is applied at the transmitter,
although IDARF performs slightly better. When the
modulation used is 64QAM, IDAR performs better than DAR
without filter at the transmitter and they both have similar

__________________________________________________________________________________________
behavior. IDARF, on the other hand, performs clearly better
than DAR and IDAR and its behavior is closer to the case
when no clipping is applied than to IDAR and DAR. When the
methods are analyzed for different clipping ratios, the
simulations show that when the filter is used in the transmitter
and for CR < 7 dB, IDARF performs better than all the other
methods, for higher CR they all perform the same. When
comparing all methods without filter in the receiver, IDARF
performs better than IDAR and DAR for CR < 4 dB, for
higher CR their performances are very similar. The filter is an
important element in OFDM systems since it reduces the out-
of-band radiation and ICI caused by clipping. As seen in the
simulations, when the filter is used in the transmitter, the
method that better performs in the receiver is IDARF.
Therefore, when clipping is applied, the better solution in
order to mitigate the noise that it introduces is to use the FFT
filter together with the IDARF method.
REFERENCES
[1] Jean Armstrong. “New OFDM Peak-to-average Reduction
Scheme”. In IEEE Vehicular Technology Conference, Rhodes,
Greece, 2001
[2] Pisit Boonsrimuang, Kazuo Mori, Tawil Paungma, and
Hideo Kobayashi. “Proposal of Clipping and Inter-modulation
Noise Mitigation Method for OFDM Signal in Non-linear
Channel”. IEICE Trans. Commun., 2005.
[3]Khaled Fazel and Stefan Kaiser. Multi-Carrier and Spread
Spectrum Systems. Wiley,2003.
[4]Hiroshi Harada and Ramjee Prasad. Simulation and
Software Radio for Mobile Communications.Artech House,
2003.
[5]Dukhyun Kim and Gordon L. Stuber. “Clipping Noise
Mitigation for OFDMby Decisionaided Reconstruction”. IEEE
Communication Letters, 1999.
[6]Xiaodong Li and Leonard J. Cimini. “Effects of Clipping
and Filtering on the Performance of OFDM”. IEEE
Communication Letters, 1998
[7]Ye (Geoffrey) Li and Gordon L. Stuber. Orthogonal
Frequency Division Multiplexing for Wireless
Communications. Springer, 2006.
[8]Ramjee Prasad. OFDM for Wireless Communication
Systems. Artech House, 2004
[9]Henrik Schulze and Christian Luders. Theroy and
Applications of OFDM and CDMA. Wideband Wireless
Communications. Wiley, 2003
[10]H. Sizun. Radio Wave Propagation for
Telecommunication Applications. Springer,2003
BIOGRAPHIES
Hemant Choubey, he has received the B.E.
degree in Electronics and communication
engineering from Rajiv Gandhi Technical
University Bhopal, in 2009. He is currently
pursuing M.E degree in Digital Communication
from RGTU Bhopal.

Comparison of various noise mitigation technique used

More Related Content

What's hot (18)

Viewers also liked (20)

Similar to Comparison of various noise mitigation technique used (20)

More from eSAT Publishing House (20)

Recently uploaded (20)

Comparison of various noise mitigation technique used